Terminal for electrical apparatus with conductors cooled down to a low temperature

ABSTRACT

A high voltage-resistant, current feed terminal to a superconductor cooled down to a low temperature, with its connected normal conductor being provided with a heat exchanger. Coolant input lines and coolant output lines each form spirals having turns which surround the normal conductor and which are designed as voltage dividers. The spiral-shaped design has the advantage that the coolant line can take up a high potential gradient. The voltage divider causes a uniform division of the voltage in such a manner that the breakdown voltage of the coolant is not reached.

United States Patent [1 1 Kohler et al.

TERMINAL FOR ELECTRICAL APPARATUS WITH CONDUCTORS COOLED DOWN TO A LOWTEMPERATURE Inventors: Hubert Kohler, Eltersdorf; Fritz Schmidt,Erlangen, both of Germany Assignee: Siemens Aktiengesellschaft, Munich,

Germany Filed: Nov. 2, 1972 Appl. No.: 303,060

Foreign Application Priority Data Oct. 9, 1973 [56] References CitedUNITED STATES PATENTS 3,695,057 10/1972 Moisson-Frankhauser 174/15 R3,701,944 10/1972 Amalric l74/ll BH 3,522,361 7/1970 Kafka 174/15 CPrimary ExaminerBernard A. Gilheany Assistant Examiner-A. T. GrimleyAtt0mey-Hugh A. Chapin et a1.

[57] ABSTRACT A high voltage-resistant, current feed terminal to asuperconductor cooled down to a low temperature, with its connectednormal conductor being provided with a heat exchanger. Coolant inputlines and coolant output lines each form spirals having turns which sur-N 17, l97l 0v Germany p 21 57 125 5 round the normal conductor and whichare designed US Cl H "4/15 BH I74/DlG 6 323/44 F as voltage dividers.The spiral-shaped design has the CL" HOIV 11/00 advantage that thecoolant line can take up a high po- Field of BH l2 BH tential gradient.The voltage divider causes a uniform 174/15 BH 6 division of the voltagein such a manner that the breakdown voltage of the coolant is notreached.

17 Claims, 2 Drawing Figures II/ A I II II 12 E 51 21 9 i w I j 7 22 23U UU 5 25 2s 1 32 as 5 3 r 2 11 ES\ 5;? 1 r y I 13 L i; 15 .0 11 L2 4E TT TERMINAL FOR ELECTRICAL APPARATUS WITH CONDUCTORS COOLED DOWN TO A LOWTEMPERATURE BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention relates to a current feed terminal for electricalapparatus with conductors cooled down to low temperatures, and moreparticularly to superconductors, the end of which is connected to anormal conductor which is provided with a heat exchanger.

2. Description of the Prior Art In electrical apparatus withsuperconductors, for instance in superconducting cables, coils ormachines, electric current must frequently be fed to the superconductor,which is cooled down to a temperature below its critical temperature,from a point which is at a higher temperature, particularly at roomtemperature. As the superconductor would lose its superconductivityalready far below room temperature, electrically normal conductingmaterial, for instance, aluminum or copper, is used to bridge thetemperature difference, which is connected with the superconductor at apoint which is kept at a temperature below the critical temperature ofthe superconductor.

In superconducting single or three-phase cables, the conductors can bedesigned, as is well known, as concentric tubes, the inner tube of whichis used as the outgoing conductor, and the outer tube as the returnconductor. The three phases are interlinked outside of the cable proper.This arrangement has the advantage that the electromagnetic field isonly between the inner and the outer conductor. The superconductors fora-c current of 50 Hz. are preferably made of pure metals, such asniobium or lead. These have only very low hysteresis losses so long as apredetermined critical field strength is not exceeded. Such cables withconcentric conductors can be operated at high voltages of, for example,200 kV and more. Since superconducting cables are used in the range ofabout 1 GW and up, the terminals must, therefore, be designed not onlyfor high voltage, but also for high currents of about kA and up.

As the coolant for superconductors, only helium can be considered forall practical purposes. Required is, therefore, not only a current feedfor high voltage and high currents, which is designed for a temperaturegradient from room temperature to below the critical temperature of thesuperconductor, but also a voltageresistant feed-in of the coolingmedia, as the latter are at the potential of the conductor and,therefore, at high voltage.

In the German Published Pat. application No. 1,655,940, there isdisclosed a current feed for a superconducting cable wherein heatexchangers are provided which are in direct contact with the normalconductor. This type of construction, however, is suitable only formedium conductor voltages because of the relatively short length of thecoolant feeds.

In the publication Conference on Low Temperatures, Electric and Power,London, March 24, to 26, 1969, p. 254, 255 and 259, a current feed to asuperconductor is disclosed which has heat exchangers for the normalconductor, which are separated from the normal conductor by insulatingmaterials. For a high voltage resistant terminal, accordinglyhigh-quality electrical insulating materials must be used. Suchinsulating materials, however, are poor heat conductors. With this typeof design, one, therefore, obtains relatively poor heat removal withincreasing voltage and high current.

Although one can use, in such current feeds for conductors which are atcryogenic temperature, the rising gas which is generated by the boilingof the cooling liquid and which is warmed up accordingly, problems canarise in the cooling of the current feed. Specifically,

problems can arise in the cooling of electrical apparatus withsuperconductors, particularly superconducting a-c cables, due to agaseous-liquid two-phase mixture as the evaporation which takes placealong the cable leads to a steadily increasing flow velocity of the gas.This gas carries the drops of the liquid along and thereby preventsuniform cooling.

The use of evaporating cooling media with separate cooling units forcooling the current feed has, furthermore, the disadvantage that only arelatively small voltage difference can be bridged, since evaporatingcooling media in the gaseous state at normal pressure have asubstantially reduced breakdown voltage, as is well known. At normaltemperature, evaporated helium has, for example, a breakdown voltage ofonly about 3 kV/cm, which is even lower for flowing gas.

SUMMARY OF THE INVENTION It is an object of the invention to provide acurrent feed terminal for electrical apparatus with superconductors,which is suitable for high currents and a high conductor potential atcryogenic temperatures.

The above object is achieved by the present invention which provides aninput and output of the coolant to the heat exchanger consisting ofelectrically insulating material. The input and output are each designedas a spiral, the turns of which surround the normal conductor, and withthe input and output, respectively, associated with an electricalresistance arrangement which constitutes a voltage divider whichproduces a voltage gradient in the cooling liquid which is smaller thanthe breakdown voltage for corresponding length units. The resistancearrangement can consist simply of a semiconductor layer at the insidesurface of the cooling tube. This semiconductor layer produces aconstant voltage gradient and, therefore, a steady voltage curve overthe distance of the coolant input or output line. However, a resistancearrangement can also be provided which subdivides the total voltage overthe length of the tube into preferably equal steps. For this purpose, aresistive, a resistive-capacitive or a capacitive voltage divider can beprovided.

For cooling the terminal, a one-phase coolant, for instance, liquidhelium, can preferably be used, which removes the heat from a heatexchanger which is disposed on the normal conductor in the immediatevicinity of the superconductor connection. For controlling thetemperature of the normal conductor in steps from the temperature of thesuperconductor to room temperature, it may be advisable to arrangeseveral further heat exchangers, preferably in direct contact with thenormal conductor, the heat of which is conducted away by other coolingmedia. For instance, the heat exchanger adjacent to the superconductorconnection can be cooled with helium, a further one with hydrogen and athird one with nitrogen.

A particularly advantageous further embodiment of the terminal accordingto the invention consists of the provision that at least the input orthe output line of the coolant to the individual heat exchangers formsthe surface of a truncated cone, the axis of which is approximately thesame as the axis of the normal conductor. The outer diameter of each ofthe turns is then chosen at least approximately equal to, but preferablysomewhat larger than the inside diameter of the following, larger turnof the spiral. This arrangement has the advantage that the coolant lineforms at the same time a radiation shield for the heat which is radiatedin the direction of the axis of the normal conductor, for instance, froma corresponding feedthrough in the direction toward the superconductor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates anembodiment of a terminal for a superconductor according to theinvention, and specifically shows a longitudinal cross section through aterminal for one phase with coaxially related outgoing and returnconductor; and

FIG. 2 shows a particular design of a voltage divider used in theterminal of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, two coaxiallydisposed carriers 1 and 2 are provided, each for one superconductor. Thecarrier 1 is provided with a superconducting layer 3 on the outside, andthe carrier 2 in a similar manner with a superconducting layer 4 on theinside. The superconductor 3 may, for example, be the outgoingconductor, and the superconductor 4 the return conductor of the samephase ofa superconducting cable. In the space between the twosuperconducting layers 3 and 4 is located a suitable liquid coolingmedium 6, particularly helium, which keeps the superconductors 3 and 4below their critical temperature and, in the embodiment shown, is at thesame time used for cooling and voltage insulation. The position andspace between the superconducting layers 3 and 4 is set in known mannerby spacers 5.

Deviating from the embodiment shown, the liquid helium which serves asthe coolant can also be conducted inside the carrier 1- and outside thecarrier 2 in an additional tube. Such a design makes insulation possiblebetween the two superconductors 3 and 4 by means of a high vacuum, asuitable synthetic material or a stationary, non-flowing coolant.

An insulator 8 is arranged at the end of the concentric system with thesuperconductors 3 and 4. The conductor arrangement rests on a radiationshield 10 by means of supports 9. Radiation shield 10 is supported bymeans of thermal insulation 11 in an outer protective pipe 12. To theprotective pipe 12 is connected a vacuum vessel 13, which is providedwith a connection 14 for a high-vacuum pump, not shown, and whichcontains the entire terminal with a normal conductor 20. The returnconductor 4 is enlarged within the vacuum vessel 13 via a conical,hollow coolant input 18 to form a cylindrical conductor 15, which isbrought out by means of a high-vacuum seal 16. The coolant input 18 isconnected to a supply line 17.

The current feed to the outgoing conductor 3 is established via anormal-conducting bushing 19, which is at normal temperature and highvoltage and which may optionally be provided with a separate coolingarrangement. Bushing 19 may consist in a known manner of an insulatorwhich must be designed on the outside for adequate dielectric strengthin air and on the inside for high vacuum.

Normal conductor 20 may consist of copper or aluminum and may be a solidconductor inside the bushing 19. On the normal conductor 20 are situatedthree heat exchangers 21, 22 and 23, which are in direct contact withthe normal conductor 20, which in this region may preferably consist offine wires, a thin woven screen or thin individual cylinders, which aredesignated with numeral 24. In some cases, it may also be advisable tomake the normal conductor in the region of the heat exchangers 21 to 23of a different material, preferably nickel. The gauge of such thinindividual conductors 24 is determined by the depth of penetration ofthe current at the frequency of the conductor and by the respectivetemperature. The different heat exchangers 21 to 23 can preferably becooled with different cooling media for establishing the temperaturegradient between the superconductors 3 and 4 and the normal temperatureoutside the bushing 19. Liquid helium can be fedto the heat exchanger 21via a coolant supply line 26 made in the form of a spiral, which isdischarged via a coolant discharge line, the spiral of which forms atruncated cone. The truncated cone is designed so that the outsidediameter of a smaller turn is at least approximately as large, andpreferably equal to or larger than the inside diameter of the adjacentlarger turn. In this embodiment, the coolant discharge line 28 serves atthe same time as a radiation shield for heat rays which enterapproximately in the direction of the normal conductor 20 into thevacuum vessel 13 and are directed toward the superconductors 3 and 4.The next heat exchanger 22 can be cooled with hydrogen via a coolantsupply and discharge line 30 and 32, respectively, designed in a similarmanner. For the third heat exchanger 23, nitrogen may be provided as thecoolant which is fed in via a supply line 34 and is discharged via aline 36. In this design, the normal conductor is cooled down in steps inthe region of the three heat exchangers from room temperature via thetemperature of the liquid nitrogen with about 77 K and the temperatureof the liquid hydrogen of about 20 K to the temperature of the liquidhelium with about 4 K, so that the superconductor connection 7 is belowthe critical temperature of the superconducting materials.

Also, the cylindrical return conductor 15, which is situated in thevacuum vessel 13 within a radiation shield 38 cooled with nitrogen orhydrogen, can advantageously be provided with heat exchangers which aredesignated in the FIG. 1 with numerals 40 and 42 and to which can be fedhydrogen or nitrogen as the coolant via coolant feed and dischargelines, which are shown but not further marked. The heat exchanger 40 canadvantageously be cooled with hydrogen and the heat exchanger 42 withnitrogen.

In order to obtain a path as long as possible for the coolant feed linesin at the same time a small space, these lines are led in spiral fashionfrom the inside to the outside. Such coolant lines 26, 28, 30, 32, 34and 36 can preferably be designed at the same time as a resistive,resistive-capacitive or as a capacitive voltage divider, as is shown inFIG. 2. Since a capacitive voltage divider generates practically nothermal losses, it appears particularly well suited for an a-c terminal.In the FIG. 2, one of such coolant lines, which may consist of anelectrically non-conducting material such as ceramic, plastic, glass orquartz, is designated with numeral 44. Coolant line 44 is subdivided byring-shaped connecting pieces 46 serving as potential rings arrangedinto individual potential steps. The potential rings 46 may consist ofan electrically highly conducting material, such as copper of nickel.The potential rings 46 are on the one hand in direct contact with thecooling liquid in the cooling tube 26, and are on the other hand inelectrically conducting contact with an outer metal sleeve 48. The twoends of the metal sleeve 48 are each folded over to form a hollow ring50, in a so-called Rogowski profile. The profiles have a predeterminedmutual distance A, which determines the electrode capacity. Thisdistance determines the magnitude of the capacitive current through thevoltage divider. As the flash-over voltage in a vacuum is high, this gapA can be kept relatively small. Through the spacing of the potentialrings 46, the voltage drop at the individual stages of the voltagedivider is determined. This voltage drop is chosen so that the breakdownvoltage of the cooling liquid is not reached with a margin of safety.For the voltage resistance of the surface, the relatively long path ofan insulating-tube section 52 is available in each case, which is givenby the distance of two succeeding potential rings 46.

It is to be understood that, besides the design of the coolant feed anddischarge as a voltage divider shown in the embodiment according to FIG.2, other configurations are also possible to achieve voltage resistance.

In the example of the embodiment, the invention has been explained withreference to a terminal for a superconducting cable. However, thecurrent feed according to the invention can be used generally forelectrical apparatus with deep-cooled conductors.

Although the above description is directed to the preferred embodimentof the invention, it is noted that other variations and modificationswill be apparent to those skilled in the art, and, therefore, may bemade without departing from the spirit and scope of the presentinvention.

What is claimed is:

l. A current feed terminal for electrical apparatus having conductorscooled down to a low temperature, comprising:

a normal conductor connected to the end of said terminal;

heat exchanger means connected to said normal conductor; and

input and output lines for the coolant to said exchanger, said input andoutput lines consisting of electrically insulating material with each ofsaid lines designed as a spiral, the turns of which surround said normalconductor, and each of said input and output lines having associatedtherewith an electric resistance arrangement which constitutes a voltagedivider producing a voltage gradient in the cooling liquid which issmaller than the breakdown voltage of the coolant for correspondinglength units of said input and output lines.

2. Terminal according to claim 1, wherein said resistance arrangementconsists of a semiconductor layer on the inside surface of said input oroutput line, respectively.

3. Terminal according to claim 1, wherein said coolant is a one-phasecoolant.

4. Terminal according to claim 1, wherein at least the spiral of saidinput or output lines forms the surface of a truncated cone.

5. Terminal according to claim 4, wherein the inside diameter of each ofthe turns of said spiral is equal to or smaller than the outsidediameter of the adjacent, smaller turn.

6. Terminal according to claim 1, wherein said voltage divider comprisesa capacitive voltage divider.

7. Terminal according to claim 6, wherein at least one of said coolantinput lines comprises individual sections which are connected with eachother via annular intermediate pieces of electrically conductingmaterial, and each of said intermediate pieces is connected with asleeve of electrically conducting material which surrounds said coolantline, the ends of said sleeve being folded over to form a hollow ringwhich has a predetermined distance from the similarly formed end of theadjacent sleeve.

8. Terminal according to claim 1, wherein said heat exchanger means isarranged in a high-vacuum vessel.

9. Terminal according to claim 8, further comprising a radiation shieldbetween said heat exchanger means and the wall of the high-vacuumvessel.

10. Terminal according to claim 9, wherein a separate coolingarrangement is provided for said radiation shield.

11. Terminal according to claim 1, wherein liquid helium is provided asthe cooling medium for said heat exchanger means.

12. Terminal according to claim 1, wherein gaseous helium underoverpressure is provided as the cooling medium for said heat exchangermeans.

13. Terminal according to claim 1, wherein a second heat exchanger meansis further provided for said normal conductor.

14. Terminal according to claim 13, wherein hydrogen is provided as thecooling medium for said second heat exchanger means.

15. Terminal according to claim 14, wherein a third heat exchanger meansis further provided for said normal conductor.

l6. Terminal according to claim 15, wherein liquid nitrogen underoverpressure is provided as the cooling medium for said third heatexchanger means.

17. Terminal according to claim 1, wherein at least one of said inputand output lines comprises a tubular line which is coated on the insideor outside with a semiconductor, the electric conductivity of whichchanges over the length of the tube in steps, or gradually.

1. A current feed terminal for electrical apparatus having conductorscooled down to a low temperature, comprising: a normal conductorconnected to the end of said terminal; heat exchanger means connected tosaid normal conductor; and input and output lines for the coolant tosaid exchanger, said input and output lines consisting of electricallyinsulating material with each of said lines designed as a spiral, theturns of which surround said normal conductor, and each of said inputand output lines having associated therewith an electric resistancearrangement which constitutes a voltage divider producing a voltagegradient in the cooling liquid which is smaller than the breakdownvoltage of the coolant for corresponding length units of said input andoutput lines.
 2. Terminal according to claim 1, wherein said resistancearrangement consists of a semiconductor layer on the inside surface ofsaid input or output line, respectively.
 3. Terminal according to claim1, wherein said coolant is a one-phase coolant.
 4. Terminal according toclaim 1, wherein at least the spiral of said input or output lines formsthe surface of a truncated cone.
 5. Terminal according to claim 4,wherein the inside diameter of each of the turns of said spiral is equalto or smaller than the outside diameter of the adjacent, smaller turn.6. Terminal according to claim 1, wherein said voltage divider comprisesa capacitive voltage divider.
 7. Terminal according to claim 6, whereinat least one of said coolant input lines comprises individual sectionswhich are connected with each other via annular intermediate pieces ofelectrically conducting material, and each of said intermediate piecesis connected with a sleeve of electrically conducting material whichsurrounds said coolant line, the ends of said sleeve being folded overto form a hollow ring which has a predetermined distance from thesimilarly formed end of the adjacent sleeve.
 8. Terminal according toclaim 1, wherein said heat exchanger means is arranged in a high-vacuumvessel.
 9. Terminal according to claim 8, further comprising a radiationshield between said heat exchanger means and the wall of the high-vacuumvessel.
 10. Terminal according to claim 9, wherein a separate coolingarrangement is provided for said radiation shield.
 11. Terminalaccording to claim 1, wherein liquid helium is provided as the coolingmedium for said heat exchanger means.
 12. Terminal according to claim 1,wherein gaseous helium under overpressure is provided as the coolingmedium for said heat exchanger means.
 13. Terminal according to claim 1,wherein a second heat exchanger means is further provided for saidnormal conductor.
 14. Terminal according to claim 13, wherein hydrogenis provided as the cooling medium for said second heat exchanger means.15. Terminal according to claim 14, wherein a third heat exchanger meansis further provided for said normal conductor.
 16. Terminal according toclaim 15, wherein liquid nitrogen under oveRpressure is provided as thecooling medium for said third heat exchanger means.
 17. Terminalaccording to claim 1, wherein at least one of said input and outputlines comprises a tubular line which is coated on the inside or outsidewith a semiconductor, the electric conductivity of which changes overthe length of the tube in steps, or gradually.